Magnetic memory devices (HDDs) mainly used in computers to record and reproduce information are recently beginning to be used in various applications because they have large capacities, inexpensiveness, high data access speeds, a high data retaining reliability, and the like, and they are now used in various fields such as household video decks, audio apparatuses, and automobile navigation systems. As the range of applications of the HDDs broadens, demands for large storage capacities increase, and high-density HDDs are more and more extensively developed in recent years.
As a magnetic recording method of presently commercially available HDDs, a so-called perpendicular magnetic recording method is recently most frequently used. In the perpendicular magnetic recording method, magnetic crystal grains forming a magnetic recording layer for recording information have the axis of easy magnetization in a direction perpendicular to a substrate. The axis of easy magnetization is an axis in the direction of which magnetization easily points. In a Co-based alloy, the axis of easy magnetization is the axis (c-axis) parallel to the normal to the (0001) plane of the hcp structure of Co. Even when the recording density is increased, therefore, the influence of a demagnetizing field between recording bits is small, and the medium is magnetostatically stable. A perpendicular magnetic recording medium generally includes a substrate, a soft magnetic underlayer for concentrating a magnetic flux generated from a magnetic head during recording, a nonmagnetic seed layer and/or nonmagnetic underlayer for orienting the magnetic crystal grains of a perpendicular magnetic recording layer in the (0001) plane and reducing the orientation dispersion, the perpendicular magnetic recording layer containing a hard magnetic material, and a protective layer for protecting the surface of the perpendicular magnetic recording layer. A film mainly used as the existing perpendicular magnetic recording layer is a multilayered film including a granular film type recording layer having a so-called granular structure in which magnetic crystal grains are surrounded by a grain boundary region made of a nonmagnetic material, and a continuous film type recording layer having a continuous-film-like structure not having a clear grain/grain boundary structure.
The granular film type recording layer has a structure in which magnetic crystal grains are two-dimensionally, physically isolated by a nonmagnetic grain boundary region, so the magnetic exchange interaction acting between the magnetic grains reduces. This makes it possible to reduce the transition noise of the recording/reproduction characteristics, and decrease the limit bit size. On the other hand, since the exchange interaction between the grains is reduced in the granular film type recording layer, the dispersion of a magnetization switching field (SFD) often increases in accordance with the composition of the grains and the dispersion of the grain diameter. This increases the transition noise or jitter noise of the recording/reproduction characteristics. Accordingly, it is impossible to obtain favorable recording/reproduction characteristics by using the granular film type recording layer alone.
By contrast, the continuous film type recording layer does not have a clear grain/grain boundary separated structure unlike the granular film type recording layer, so a relatively strong exchange interaction two-dimensionally, almost uniformly acts between magnetic crystal grains. When stacking this recording layer on the above-described granular film type recording layer, an exchange interaction with an appropriate magnitude can uniformly be exerted between the magnetic crystal grains in the granular film type recording layer through the continuous film type recording layer. This makes it possible to suppress the SFD described above, and remarkably improve the recording/reproduction characteristics when compared to those when using the granular film type recording layer alone. Note that this effect can be obtained only when the magnetic interaction is sufficiently acting between the granular film type recording layer crystal grains and continuous film type recording layer crystal grains, and no satisfactory effect is obtained if the interaction deteriorates as will be described later.
As described previously, the lower limit of the recording bit size strongly depends on the magnetic crystal grain diameter of the granular film type recording layer. To increase the recording density of the HDD, therefore, it is necessary to decrease the grain diameter of the granular film type recording layer. An example of methods reported as a method of decreasing the grain diameter of the granular film type recording layer is to decrease the grain diameter of an underlayer by, e.g., improving a seed layer or using a granular film as the underlayer, thereby decreasing the grain diameter of a granular film type recording layer to be stacked on the underlayer. Accordingly, the grain diameter of the granular film type recording layer can be decreased by using an underlayer having a small crystal grain diameter.
On the other hand, the present granular film type recording layer has a columnar structure in which one magnetic crystal grain epitaxially grows on one crystal grain of a nonmagnetic underlayer. However, the grain diameter of the magnetic crystal grains in the film plane is not constant in the direction of film thickness, but tends to decrease as the grains grow. The number of grains is constant in the direction of film thickness. On the surface of the granular film type recording layer, therefore, the area of the grain boundary region increases and the total area of the magnetic crystal grains decreases when compared to those in the initial growth portion. In other words, the packing ratio of the magnetic crystal grains decreases. In the granular film type recording layer in which the magnetic crystal grain diameter is decreased as described above, the grain packing ratio on the film surface significantly decreases from that of the conventional granular film type recording layer.
Accordingly, when stacking the continuous film type recording layer on the granular film type recording layer having the decreased magnetic crystal grain diameter, the contact area between the upper and lower magnetic crystal grains in the granular film type recording layer/continuous film type recording layer interface decreases, because the magnetic crystal grain packing ratio on the surface of the granular film type recording layer is very low. Consequently, the exchange interaction between the magnetic crystal grains in the two layers deteriorates, and the above-described magnetic characteristic adjusting function significantly degrades. This poses the problem that the recording/reproduction characteristics deteriorate because it is impossible to sufficiently suppress the SFD.